Advances in Low NOx Residual Oil Firing for Boilers

Scott A. Drennan, Mandayam Vijay, Coen Company, Inc.

ABSTRACT:

Residual oil continues to be the fuel of choice for many industrial steam users while environmental regulations allow its use. However, new emissions regulations on NOx and SOx are requiring substantial improvements in emissions performance or it will be regulated out of existence. This paper discusses the results and effectiveness of two applications of low NOx design modifications to residual oil firing industrial boilers. A comparison of the low NOx approaches required and results obtained for a package and field erected boiler are presented. Previous development in low NOx residual oil firing indicate that significant NOx reduction can be obtained through spray pattern modification to produce a staged spray flame with rich and lean zones. Industrial package boilers have specific performance criteria that impede the effectiveness of such low NOx techniques. Factors such as smaller furnaces, higher firing densities, lack of air preheat, widened operating envelope, simple controls, and variable fuel quality make the industrial package boiler a more challenging low NOx environment than a multiple burner field erected or utility boiler. The greatest NOx reductions are obtained in field erected boilers with low firing densities as a result of the ability to stage the combustion. The variability in fuel characteristics such as viscosity, distillation curve, carbon residue, and ash composition limits the potential emissions reduction and maintaining stable combustion.

Figure 1, The Multiple Venturi Atomizer

INTRODUCTION:

Heavy residual oil is the fuel of choice for industrial and utility boiler owners where natural gas and coal are not continuously available. Many regions within the United States have been designated as Ozone Non-Attainment areas under the recently revised National Ambient Air Quality Standards (NAAQS) and are under increasing pressure to reduce NOx emissions. NOx emissions reductions can be obtained by (1) switching to an alternate fuel with less propensity for NOx production such as natural gas, (2) installation of flue gas treatment systems such as SCR and SNCR, and (3) installation of low NOx burner equipment. The first option is not possible where natural gas is unavailable, such as in the Northeast United States. The second option, SCR or SNCR, is costly and prohibitive to many industrial boiler owners. The most cost effective option to reduce NOx emissions from residual oil is the installation of low NOx burner equipment. This paper compares the results of two recent applications where low NOx techniques are utilized in a field-erected and a package water tube boiler.

Multiple Venturi MV Atomizer: COEN’s predominant atomizer of choice for boiler applications is the Multiple Venturi (MV) atomizer. The MV atomizer is an inside mix, twin fluid atomizer used for the combustion of liquid fuels from light oil up to heavy asphalt or pitch which require heating prior to atomization (see Figure 1). Liquid fuel passes through the central fuel tube, surrounded by the steam tube, until reaching the multiple venturi mixer where a two-phase mixture is produced. This mixture is forced through a nozzle body to increase its momentum prior to exiting through a plain-jet cap. The cap design of the MV allows great flexibility for flame shaping through the cap drilling pattern. Four different sizes of the MV atomizer are used to achieve capacity ranges from 1 to 40 gallons per minute fuel flow.

BACKGROUND:

NOx is the term typically used to refer to NO, and NO2 formed by combustion. Man-made NOx sources are a small portion of the global NOx; however, they tend to be concentrated in urban areas resulting in excessive production of ozone which is a known lung irritant. The two sources of nitrogen for the formation of NOx in industrial combustion are thermal and fuel. The thermal and fuel NOx formation mechanisms are outlined below.

The thermal NOx mechanism is initiated with the high temperature disassociation of atmospheric nitrogen molecules. Atmospheric nitrogen has a triple atomic bond and is extremely stable. Atmospheric nitrogen will only decompose at the very high temperatures within the flame. The resultant nitrogen atoms are unstable and will precede rapidly to form either NO or a new nitrogen molecule. Thermal NOx is therefore very temperature dependent. For fuel lean combustion, the Zeldovich mechanism is accepted as an approximation of NOx formation.

N2 + O ® NO + N
(RATE LIMITING)
(1)

N + O2 ® NO + O
(FAST)
(2)

In staged combustion, thermal NOx is minimized by:

1. Reducing Peak Flame Temperatures,

2. Reducing the Oxygen Levels at the Peak Flame Temperatures,

3. Reducing the Time of Exposure to the Peak Flame Temperatures.

For nitrogen containing fuels such as residual oil, the oxidation of nitrogen that is organically bound to the fuel molecule becomes the primary source of total NOx emissions. For this reason, overall NOx reduction efforts are focused on reducing the conversion of fuel bound nitrogen (FBN) to NOx. Recall that in thermal NOx formation, the nitrogen source is the extremely stable (triple atomic bond) atmospheric nitrogen. The source of nitrogen for fuel NOx is actually single bonded directly to the fuel oil molecule. As the fuel burns, all bound nitrogen disassociates to form highly unstable nitrogen radicals. These nitrogen radicals will either be oxidized to form NOx, or bond with another nitrogen radical to form N2 (atmospheric nitrogen.) To minimize the oxidation of FBN, we try to delay mixing of stoichiometric air. The less oxygen available to the nitrogen radicals, the more the reaction will favor the formation of N2. The desired results are long, narrow flames with fuel rich zones to inhibit the conversion of FBN to NOx. Ideally, the combustion reaction will not be complete until just before the gasses turn into the convection tube bank.

Thermal NOx and fuel NOx are both being formed in the combustion reaction. Although the formation mechanisms are widely different, the minimization techniques are similar. The long fuel rich flame established to minimize FBN oxidation also lowers the flame temperature and reduces thermal NOx. The delayed mixing of stoichiometric air also reduces the amount of atmospheric nitrogen available at the peak temperature zones at the base of the flame

Residual oil contains a significant amount of fuel bound nitrogen which can contribute more than 50% of the total NOx emissions. Low NOx residual oil techniques require the reduction of the conversion of fuel bound nitrogen to NOx in addition to thermal NOx.

Figure 2, Effects of FBN on NOx Emissions with 100% Conversion

Figure 2 illustrates the great potential for fuel bound NOx generation and the typical effects of FBN on NOx emissions with a 100% conversion of FBN to NOx.1,2 Recent field demonstrations have shown that rich-lean spray staging has great potential for reducing fuel NOx production from residual oil flames. 3 The greatest limiting factor to rich-lean staging in residual oil spray flames is the generation of particulate matter. As the oil droplet vaporizes in the furnace, a hollow cenosphere of carbonaceous material is formed whose diameter and mass are dependent upon the original oil droplet diameter and the quantity of coke forming carbon in the original fuel.4,5 Figure 2: Effects of FBN on NOx Emissions with 100% Conversion

Any successful low NOx atomizer must meet current particulate requirements. By using the Coke Formation Index (CFI) relationship as detailed in Equation 3, the expectant coke particle diameter (Dc) can be calculated from the original droplet diameter (Do). In this equation, t is the coke particle shell thickness and rc and ro are the densities of the coke particle and the initial droplet, respectively. The oxidation of these particles is based on surface combustion of carbon and the expected burnout for various particle sizes at typical furnace oxygen and temperature conditions.

Figure 3, Droplet Sizes

Figure 3 illustrates how the smaller droplets generate small coke particles which have sufficient residence time to oxidize completely. Figure 3 also demonstrates that the dominant particulate emissions arise from the large original droplet sizes. This analysis demonstrates that attempts to produce fuel rich zones of combustion with large droplet diameters results in increased particulate emissions. Figure 3: Predicted Particulate Mass Emission Based on Original Droplet Size Distribution

APPROACH:

It is important that any low NOx emissions approach taken not only achieve the specific emissions goals, but also maintains acceptable operational features required by the installation. Table 1 illustrates the required performance features of firing rate and turn down. However, the combustion system must also provide repeatable emissions performance which can be maintained in compliance at all times on specified rolling averages when Continuous Emissions Monitors (CEM) are used. The combustion system must also be reliable by providing safe and stable performance during ignition, shut down, and normal maintenance procedures like gun cleaning, soot blowing, fuel changeover.

OPERATIONAL EMISSIONS
  • Maximum Firing Rate
  • Turn Down
  • Stability
  • Repeatability
  • Reliability


  • NOx
    • Fuel
    • Thermal
  • Opacity and Smoke
  • Particulate
  • CO

Table 1, Burner Performance Considerations

The following is a list of the low NOx approaches utilized by COEN burners for residual oil firing applications:

· Air Staging with NOx or Overfire Air Ports: The fuel is deprived of oxygen in the early stages in the flame when the FBN is evolving from the fuel. This is physically done by using NOx or overfire air ports where some of the combustion air is diverted away from the burner throat. The throat is operated in a sub-stoichiometric fashion and the resulting oxygen depletion inhibits the conversion of FBN to NOx. The critical issues for effectiveness of air staging with ports are (1) effective oxidation of particulates reduced opacity and (2) flame stability at sub-stoichiometric levels.

· Spray (or Fuel) Staging: This technique works in much the same way as NOx or overfire air ports by creating areas of heavy and light fuel spray for reductions in FBN conversion. The heavy areas of spray increase the flame evaporation zone and shield oxygen from the center of these dense areas of fuel vapor. This inhibits the conversion of FBN to NOx by oxygen depletion. The light fuel spray sections rapidly evaporate and burn increasing stability for the flame.

· Flue Gas Recirculation: FGR is a well proven thermal NOx reduction technique where the oxygen content of the combustion air is decreased by dilution with flue gases. The effects of FGR on highly staged residual oil flames are significantly less than with non-staged flames as most of the excess oxygen is effectively removed by the air or fuel staging described above. However, some reductions in thermal NOx are evident by the use of FGR even in staged residual oil flames. Thus, FGR is utilized on very low NOx residual oil applications when every NOx reduction technique is required to meet the emissions limits. The ultimate degradation of flame stability is the limiting factor in the use of FGR for further reductions in NOx emissions.

The use of flame staging low NOx techniques described above do not come without increased challenges. The following is a list of the challenges resulting from staged residual oil flames:

· Increased flame volume and length which could be limited by furnace size

· Potential for increased particulate emissions as a result of reduced coke particle burn out time

· Increased opacity

· Emissions performance over the firing range

· Light off and stability

· Furnace temperature distribution

CASE STUDY RESULTS:

Description of the Case Studies:

The two residual oil firing cases considered in this paper illustrate the differences in low NOx approaches for package and field erected boilers at emissions levels commonly required. The pertinent emissions and performance requirements for each of the two boilers are listed in Table 2.

PACKAGE BOILER FIELD ERECTED BOILER
PERFORMANCE


  • 150,000 lb/hr Capacity
  • No Air Preheat
  • 8:1 Turn Down
  • 2 Burners (1 over 1)
  • 90 MBtu/hr Burner Capacity

EMISSIONS

  • 0.25 lb/MBtu NOx
  • 0.08 lb/MBtu Particulate
  • 20% Opacity (10% req’d)
  • CEM
PERFORMANCE


  • 300,000 lb/hr Capacity
  • 700°F Air Preheat
  • 6:1 Turn Down
  • 6 Burners (3 over 3)
  • 60 MBtu/hr Burner Capacity

EMISSIONS

  • 0.3 lb/MBtu NOx
  • 0.12 lb/MBtu Particulate
  • 20% Opacity
  • CEM

Table 2, Comparison of Performance and Emissions Requirements


Figure 4, Package and Field Erected Boiler Furnace Geometries

The package boiler is a newly installed, modern package boiler of conventional design. This boiler type is most susceptible to low NOx requirements as it is a new source of emissions. The boiler is only fired with residual oil and utilizes propane pilots for ignition and must meet 0.25 lb/MBtu NOx emissions. This installation utilizes two burners as a requirement by COEN in order to maintain effective flame volume control over the staged flames. A single burner would have been required if the NOx emissions were not so stringent. This boiler application is representative of many new boiler installation firing residual oil. The package boiler has a very narrow furnace and approximately 0.3 seconds of flue gas residence time in the furnace at full fire, see Figure 4. All emissions are required to be met on 6 minute rolling averages on the CEM. The package boiler requires opacity emissions of less than 20%. However, the customer requires that the opacity be kept less than 10% as a result of local resident complaints of a dirty stack when the opacity rose above 10%.

The field erected boiler case is indicative of a low NOx retrofit installation with relatively strict NOx emissions requirements of 0.3 lb/MBtu. The actual permitted emissions are higher, but the customer has several large boilers and wished to decrease site aggregate NOx emissions. The boiler is a large field erected six burner boiler with relatively small turn down requirements as there are multiple boilers at the site. It has a very high level of air preheat at 700°F. The boiler requires 20% opacity emissions over a 6 minute rolling average and has a CEM installed.

Low NOx Approaches: These two boilers requires vastly different low NOx design approaches to meet the emissions and performance requirements even though the NOx emissions levels are relatively similar. The low NOx approaches taken are detailed in Table 3.

The field erected boiler requires minimal additional considerations for low NOx performance as a result of the generous furnace volume and multiple burner arrangements. The burners for the field erected boiler have two sets of louvers for air swirl and flow adjustments for flame shaping. A low NOx staged spray pattern is used to control FBN conversion to NOx.

The package boiler, on the other hand, requires a complex and sophisticated combustion system approach. Both fuel and air staging are required to control NOx formation. Front wall NOx ports are employed and staged sprays are used. The installation also requires the use of selective flue gas recirculation for NOx control where the FGR is separated from the non-preheated combustion air to prevent sulfur condensation and corrosion. A sophisticated combustion control system is also required to meter the flow rates of combustion air, fuel, and FGR for NOx compliance and safety. As mentioned earlier, two burners are required to control the flame shape and maximize the NOx reduction potential of the system.

PACKAGE BOILER FIELD ERECTED BOILER
  • Deep Staging of Fuel & Air Required
  • NOx Ports· Flue Gas Recirculation
  • Staged Spray
  • Advanced Metering Controls of Air, Fuel and FGR
  • No Staging of Air Required
  • No NOx or Overfire Air Ports
  • No Flue Gas Recirculation
  • Staged Spray· Simplistic Controls
  • No Need For BOOS

Table 3, Comparison of Low NOx Approaches for the Two Boilers

Performance Results Summary: The commissioning tests of each of the case studies have been completed and each has achieved both contractual and emissions compliance. However, the start up process for each of the cases are very different. The field erected boiler started up easily only requiring a single modification to the atomizer caps for reduced particulate and reduced NOx emissions. The primary differences in the two boilers as it impacts low NOx techniques are given in Table 4.

The package boiler start up proved challenging requiring many adjustments to combustion controls and the burner system for optimum operational and emissions performance. The greatest challenge faced by the burner system is to provide sufficient flame stability at reduced excess air levels in the base of the flame for NOx reductions. Once an atomizer and burner adjustment proves to meet the strict 0.25 lb/MBtu NOx performance at full load, the performance at turn down and light off would prove problematic.

CEM Issues: The presence of a CEM on residual oil fired systems poses many challenges for the boiler owner and burner supplier. The firing of residual oil requires that provisions be made for routine maintenance procedures such as soot blowing and atomizer cap cleaning. During these procedures, it is customary to increase excess air to prevent excessive opacity excursions. This forces the NOx to go out of compliance when the excess air is increased for opacity control. Thus, it is desired to operate normally with a NOx emissions margin to account for the increased NOx during soot blowing. The package boiler application required that NOx emissions during normal operation must be kept less than 0.23 lb/MBtu in order to generate enough NOx emissions margin to keep opacity down when excess air is raised during soot blowing. With NOx performance at 0.25 lb/MBtu requiring dramatic modifications to combustion, further reductions are difficult to maintain. It is important to note that the NOx emissions on an hourly, daily and yearly average would not have gone out of compliance during soot blowing procedures. Only the specific need to meet the six minute rolling average emissions forced such operational requirements upon the combustion system.

PACKAGE BOILER FIELD ERECTED BOILER
  • Narrow and Short Furnaces
  • Lack of Air Preheat
  • Lower Residence Times (approx. 0.3 sec)
  • Wide Turn Down Requirements
  • Air Staging Difficulty
  • FGR Ineffective with Staging
  • Reduced Controls Complexity
  • Large and Wide Furnaces
  • Air Preheat
  • Long Residence Times(approx. 1 sec)
  • Narrow Turn Down Requirements
  • Opportunity for Air Staging with NOx or Overfire Air Ports
  • More Conducive to Low NOx Techniques

Table 4, Comparison of Package and Field Erected Boiler Furnaces for Low NOx Applications

CONCLUSIONS AND OBSERVATIONS:

· Significant NOx emissions reductions for boilers firing residual fuel oil can be achieved through the effective use of air staging techniques such as NOx or overfire air ports.

· Fuel staging in residual fuel oil spray flames results in reductions of both thermal and fuel NOx.

· Significant NOx reduction results are achieved in field erected boilers where generous furnace dimensions allow the increased flame volume of a highly staged residual oil flame. NOx reductions in package boilers with staged spray flames are inhibited due to flame impingement on the boiler walls and resulting in increased opacity and particulate emissions.

· It is possible to achieve 0.25 lb/MBtu NOx emissions with residual oil in boilers that have sufficient furnace volume and capacity for air staging techniques. Achieving 0.25 lb/MBtu NOx in package boilers is possible, yet problematic due to the narrow furnace, short residence time and lack of air staging capabilities.

· The presence of a CEM is also problematic on low NOx residual oil applications. The CEM requires a continuous boiler NOx emissions setting lower than the permit to allow for sufficient NOx margin to cover maintenance such as soot blowing and gun cleaning procedures.

· Flue Gas Recirculation has limited, but measurable, beneficial impact on NOx emissions from highly staged residual oil firing applications.

· The future of NOx regulations in the United States for residual oil firing in boilers is uncertain.

· Further development is required to determine the effects of droplet diameter on fuel NOx conversion in practical residual fuel oil fired boilers. Additional understanding of particulate reduction in low NOx residual oil flame is also required.

ACKNOWLEDGMENTS:

The authors would like to acknowledge the efforts of Ken Ahn, Gary Rice, Mark Clavelli, and Steve Londerville of COEN Company for their efforts in the data acquisition and analysis for this paper. Gratefully acknowledged are the tireless efforts of the COEN Company Service Engineers Joe Harter and Lee Smith for their diligent efforts in the acquisition of the field data. Also acknowledged are the kind and cooperative support received from the boiler room operators during the field trials.

REFERENCES:

1. Pershing, D.W., Cichanowicz, J.E., England, G.C., Heap, M.P., and Martin, G.B., “The Influence of Fuel Composition and Flame Temperature on the Formation of Thermal and Fuel NOx in Residual Oil Flames,” Seventeenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, pp. 715-726 (1978).

2. Beér, J.M., Jacques, M.T., Farmayan, W., and Taylor, B.R., “Fuel-Nitrogen Conversion in Staged Combustion of High Nitrogen Petroleum Fuel,” Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, pp. 101-110 (1981).

3. Drennan, S.A., Vijay, M., and Rice, G., “Low NOx Experiences Firing Residual Oil in Industrial Boilers,” Paper presented at the 1997 Spring AFRC International Symposium, Ottawa, Canada (May 1997).

4. Urban, D.L., and Dryer, F.L., “New Results on Coke Formation in the Combustion of Heavy-Fuel Droplets,” Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, pp. 1437-1443 (1990).

5. Urban, D.L., Huey, S.P.C., and Dryer, F.L., “Evaluation of the Coke Formation Potential of Residual Fuel Oils,” Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, pp. 1357-1364 (1992).